Homologues of the gene encoding the 54K (Mr 54,000) subunit of the mammalian signal recognition particle have been identified in different organisms1-5. The Escherichia coli homologue, termed ffh (for fifty-four homologue), specifies a protein (Ffh) that shares many properties with its eukaryotic counterpart, including association with mammalian 7S RNA6 and the ability to bind signal sequences specifically 7,8. Ffh also associates with E. coli 4.5S RNA, showing that it can form a ribonucleoprotein complex in prokaryotes6,9,10. These results are intriguing because extensive genetic and biochemical characterization of E. coli failed to identify a signal recognition particle-like mechanism for protein export11. Here we address this issue directly by construction of a strain in which ffh expression is arabinose-dependent. Results of depletion experiments indicate that Ffh is important in protein translocation.
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